Light emitting diode with a permanent substrate of transparent glass or quartz and the method for manufacturing the same

ABSTRACT

A method of manufacturing a light emitting diode (LED) includes growing a light emitting region on a temporary substrate, bonding a transparent substrate of glass or quartz to the light emitting region and then removing the temporary substrate. A metal bonding agent also serving as an ohmic contact layer with LED is used to bond the transparent substrate to form a dual substrate LED element which is then heated in a wafer holding device that includes a graphite lower chamber and a graphite upper cover with a stainless steel screw. Because of the different thermal expansion coefficients between stainless and graphite, the stainless steel screw applies a pressure to the dual substrate LED element during the heating process to assist the bonding of the transparent substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a division of U.S. patent application Ser. No.09/307,681, filed May 10, 1999.

FIELD OF THE INVENTION

[0002] The present invention relates to a light emitting diode with apermanent substrate of transparent glass or quartz and the method formanufacturing the same.

BACKGROUND OF THE INVENTION

[0003] U.S. Pat. Publication Nos. 5,008,718 and 5,233,204 disclosed alight emitting diode with a transparent window layer. By this kind oflight emitting diode, the crowding effect occurring in the conventionallight emitting diode is reduced, wherein the current spread to emitlight from the light emitting diode is increased. As a result, theillumination of the light emitting diode is apparently enhanced.

[0004] Moreover, U.S. Pat. Publication No. 5,237,581 and No. 4,570,172disclosed a light emitting diode having a semiconductor multilayerreflector, namely, DBR (distributed Bragg Reflector). By this lightemitting diode, the light transmitting to the substrate is reflectedbackwards so as to penetrate through the light emitting diode.Accordingly, the light illumination of the light emitting diode isincreased.

[0005] A cross sectional view of a conventional light emitting diode isillustrated in FIG. 1. The light emitting diode 100 includes asemiconductor substrate 102, a second ohmic contact electrode 101 formedon the rear side of the semiconductor substrate 102, a light generatingregion 103 formed on the semiconductor substrate 102, and a first ohmiccontact electrode 106 formed on the light generating region 103. Becauseof the current crowding effect, critical angle of the emitting light andlight absorption of the substrate, the illumination in this lightemitting diode is not suitable. The light generating region 103 isformed by a P type region and an N type region, and then the lightgenerating region 103 is grown on the gallium arsenide substrate 102.Therefore, the crystal lattice constants in most of the light generatingregion 103 are matched with that of the gallium arsenide substrate.Namely, the light emitting diode of visible light is directly fabricatedon the gallium arsenide substrate 102. However, since the energy gap ofthe gallium arsenide is 1.43eV which is smaller than that of the visiblelight and the light emitted from the diode is non-isotropic, part of thelight enters into the substrate and is absorbed by the gallium arsenide.

[0006] U.S. Pat. Publication No. 5,008,718 and No. 5,233,204 disclosed atransparent window layer structure for increasing the output light of alight emitting diode. Referring to FIG. 2, the structure of the lightemitting diode 200 is formed by a transparent window layer 204 is grownon the light emitting diode 100 shown in FIG. 1. The proper materialsuitable for the transparent window layer 204 includes GaP, GaAsP, andAlGaAs, etc., whose energy gap is larger than those of the materials inthe AlGaInP light generating region. Under this condition, the opticcritic angle can be increased and the current crowding effect is reducedso as to enhance the illumination of the light emitting diode. However,in the electric property, since the materials on the uppermost layer ofthe transparent window layer 204 and the AlGaInP light generating regionhave a hetero junction, the energy gap difference causes the positiveforward bias voltage Vf of the light emitting diode to increase. As aresult, the power loss of using the light emitting diode is increased.

[0007] The U.S. Pat. Publication Nos. 5,237,581 and 4,570,172 discloseda light emitting diode 300 with a multilayer reflecting structure, asshown in FIG. 3. The structure of FIG. 3 includes a semiconductorsubstrate 302, a lower multilayer reflector 305 formed on thesemiconductor substrate 302, a light generating region 303 formed on thelower multilayer reflector 305, an upper multilayer reflector 304 formedon the light generating region 303, a first ohmic contact electrode 306on the upper multilayer reflector 304, and a second ohmic contactelectrode 301 on the rear side of the semiconductor substrate 302. Inthis prior art light emitting diode, the lower multilayer reflector 305serves to reflect 90% of the light emitted from the light generatingregion to the light absorption substrate, while the upper multilayerreflector serves to guide light to the upper surface of the lightemitting diode. Therefore, the problem of light absorption by thesubstrate is improved, and the problem of bad illumination fromenlarging the critical angle is also improved. However, since themultilayer reflector has many hetero junctions, the effect of energy gapdifference is enlarged. As a consequence, although the aforesaid DBRstructure disclosed in U.S. Pat. Publication Nos. 5,237,581 and4,570,172 can reflect the light impinging on the substrate by the DBRstructure, the DBR has a high reflective index only for normal incidentlight (shown in D1 of FIG. 3), while for oblique incident light (such asD2, D3, and D4 shown in FIG. 3) the reflective index is very small. Thusit is only a slight improvement to the illumination of a light emittingdiode in visible light band. Whereas the DBR structure will increase thecost and difficulty about the growing of thin film epitaxial layer.

[0008] U.S. Pat. Publication No. 5,376,580 disclosed a light emittingdiode with wafer bonding, wherein a gallium arsenide substrate serves asa temporary substrate to grow a light emitting diode structure(including a confinement layer, an active layer and another confinementlayer). Then the light emitting diode structure is adhered to atransparent substrate, and the GaAs substrate is removed. Therefore, thelight absorption by substrate can be solved completely. Whereas thetransparent substrate disclosed in the aforementioned U.S. Pat.Publication No. 5,376,580 is made by GaP which is very expensive and hasan orange color. The light from LED to the substrate has a slight color.Further, in high temperature, the GaP as a transparent substrate needsto be processed for a long period of time (about 600˜700° C. for atleast one hour), which results in a bad effect to the p-n junction ofLED.

SUMMARY OF THE INVENTION

[0009] Accordingly, the object of the present invention is to provide amethod for manufacturing a light emitting diode with a permanentsubstrate of transparent glass or quartz. Transparent glass or quartz isused as a permanent substrate, and metal is employed as a bonding agent.An LED element is adhered to the transparent glass. After being adhered,an etching agent serves to remove the GaAs substrate. Therefore, theproblem of light absorption by the substrate is improved, and theproblem of the p-n junction being affected by temperature is resolvedcompletely. The illumination is doubled.

[0010] Another object of the present invention is to provide a lightemitting diode with a permanent substrate of transparent glass orquartz. The light emitting region of the light emitting diode with apermanent substrate of transparent glass or quartz is a conventionallight emitting region structure. For example, it may be a light emittingregion of dual hetero structures with an upper cladding layer/an activelayer/a lower cladding layer, a light emitting region of a single heterostructure, or a light emitting structure of a homostructure. Thepermanent substrate of transparent glass of the present invention can beapplied to all kinds of conventional light emitting region and thus ithas wide applications.

[0011] A further object of the present invention is to provide a methodfor manufacturing a light emitting diode with a permanent substrate oftransparent glass or quartz comprising the steps of selecting atemporary substrate so as to grow on an LED light emitting region on thetemporary substrate for forming an LED element; selecting a permanentsubstrate, and adhering the LED element to the permanent substrate by ametal bonding agent; removing the temporary substrate adhered by thepermanent substrate/LED element by mechanic grinding or chemicaletching; manufacturing a plane LED element with a substrate of permanentsubstrate; forming olunic contact electrodes on the plane LED element.According to the present invention, the illumination of a light emittingdiode is enhanced.

[0012] The other object of the present invention is to provide a waferholding device used in a light emitting diode, wherein two materialswith different thermal expansion coefficients provide pressures to theLED element and transparent substrate, and thus the applied force can bemeasured by a twisting spanner. The feature of the present invention isthat stainless steel screws are used to replace a quartz sleeve. Sincethe heat expansion coefficient of the stainless steel is larger thanthat of graphite, in the process of high temperature fusion, thestainless steel applies force to the clamped object.

[0013] The various objects and advantages of the present invention willbe more readily understood from the following detailed description whenread in conjunction with the appended drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross sectional view of a conventional light emittingdiode.

[0015]FIG. 2 is a cross sectional view of a conventional light emittingdiode having a transparent window layer.

[0016]FIG. 3 shows a light emitting diode having a conventionalmultilayer reflector structure.

[0017] FIGS. 4A-4D are the flow diagram of manufacturing the lightemitting diode of the present invention by adhering an LED element to atransparent glass or quartz substrate.

[0018]FIG. 5 is a cross sectional view of the LED element of oneembodiment of the present invention.

[0019]FIG. 6 shows the flow diagram of the LED element of the presentinvention being adhered to a transparent glass substrate.

[0020]FIG. 7 is a cross sectional view of the wafer holding device ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] In the present invention, at first, an LED element is depositedon a temporary substrate. Next, the LED element is adhered to atransparent glass that serves as a permanent substrate. Next, thetemporary substrate is removed so that the light emitted from the LEDelement will not be absorbed by the substrate for enhancing theillumination of the emitted light. The LED element using the technologyof the present invention is shown in FIG. 5. The flow diagram of theprocess for adhering the LED element to the transparent glass substrateis shown in FIG. 6.

[0022] The manufacturing process in the present invention for an LEDelement with a permanent substrate of transparent glass or quartzcomprises the steps of:

[0023] Selecting a temporary substrate 42 to grow an LED light emittingregion 41 on the temporary substrate 42 for forming an LED element asshown in FIG. 4A;

[0024] Selecting a permanent substrate 44, and adhering the LED elementto the permanent substrate 44 by a metal bonding agent 43 as shown inFIG. 4B;

[0025] Removing the temporary substrate 42 adhered to the permanentsubstrate 44 and LED element by mechanic grinding or chemical etchingagent as shown in FIG. 4C;

[0026] Manufacturing a plane LED element with a permanent substrate 44;

[0027] Forming ohmic contact electrodes 411 and 412 on the plane LEDelement as shown in FIG. 4D;

[0028] If the material of the metal bonding agent 43 is identical tothat of the ohmic contact electrode 411, such as an alloy of gold andberyllium (AuBe), then etching the plane LED element 41 to the metalbonding agent 43 by chemical etching and substituting the ohmic contactelectrode 411 with the metal bonding agent 43.

[0029] The aforesaid structure can further comprise the step of platinga metal reflecting layer on the rear side of the permanent substrate 44for increasing illumination. The material of the metal reflecting layeris selected from the group of indium In, tin Sn, aluminum Al, gold Au,platinum Pt, titanium Ti, and silver Ag. The temporary substrate isselected from GaAs or InP. The permanent substrate is selected fromglass or quartz. The metal bonding agent is selected from the group ofalloys of beryllium and gold (AuBe), indium In, tin Sn, aluminum Al,gold Au, platinum Pt, titanium Ti, zinc Zn and silver Ag. The etchingagent is formed from hydrochloric acid and phosphoric acid. The LEDelement may have a p/n junction or n/p junction. An etching stop layer525 as shown in FIG. 5 is formed between the LED light emitting regionand the substrate so that the substrate can be removed effectively. Thematerial of the etching stop layer is primarily formed by materialresisting the etching liquid of the substrate and is different from thatof the substrate, such as AlAs, InGaP or Al_(x)Gal_(1-x)As.

[0030] The detailed steps of manufacturing the light emitting diodeaccording to the invention are described in the Billowing:

[0031] Adhering the LED elements (41 and 42) to the glass 44 or quartzby first washing the glass 44 or quartz; placing the glass 44 or quartzinto acetone and then washing the glass or quartz by a supersonicoscillator for 5 minutes, removing the dust of the glass 44 or quartz,then washing the glass or quartz by H₂SO₄ under a temperature of 90˜100°C. for about 10 minutes in order to remove the organic objects or heavymetal on the glass 44, and then plating the metal bonding agent 43 byvapor plating or electronic gun vapor plating. This metal serves as asticky layer. In one embodiment of the present invention, the detailedstructure of the LED element is illustrated in FIG. 5.

[0032] Adhering the LED element by first washing the pollution on thesurface of the LED element, then placing the glass 44 or quartz intoacetone and then washing the glass or quartz by a supersonic oscillatorfor 5 minutes to remove dust, thereafter, removing the oxidized layer onthe surface of the LED element by diluted HF.

[0033] Adhering the washed LED element to the glass 44 or quartz platedwith metal bonding agent 43 in air or alcohol, and then placing the LEDelement and the glass 44 or quartz plated with the metal bonding agent43 to a holding device by a proper clamping device, with reference toFIG. 4A. The structure of the clamping device is shown in FIG. 7.

[0034] Thermally processing the LED elements 41 and 42 and the glass 44or quartz plated with the metal bonding agent 43 under a temperatureranging from 300˜450° C. for about 5˜10 minutes, then naturally reducingthe temperature, as shown in FIG. 4B.

[0035] Removing the temporary GaAs substrate 42 on the processed sample(LED element and the glass or quartz plated with the metal bonding agent43) by mechanic grinding or a chemical etching agent (NH₄OH:H₂O₂) asshown in FIG. 4C.

[0036] Patterning the p/n region of the LED element by an electiveetching, namely, etching a p-type (Al₀₃Ga₀₇)In₀₅P or n-type(Al₀₃Ga₀₇)In₀₅P by HCl:H₃P0 ₄. Then, the structure shown in FIG. 4D isachieved.

[0037] Forming plane electrodes 411 and 412, namely, forming ohmiccontact electrodes of p-type (Al₀₃Ga₀₇)In₀₅P or n-type (Al₀₃Ga₀₇)In₀₅P

[0038] Plating a metal reflecting layer to the rear side of the adheredtransparent glass formed as an LED element for enhancing theillumination of the LED element.

[0039] A cross sectional view of an embodiment of the present inventionis illustrated in FIG. 5. The LED element comprises a n+GaP bondinglayer 51, a light emitting region 52 and a GaAs substrate 53. Thethickness of the GaP is 0.1˜1 μm. The GaAs substrate may be an n+, p+ orSI-GaAs substrate. The light emitting region includes an n-type(Al₀₃Ga_(0.7))In_(0.5)P upper cladding layer 52 with a thickness of0.5-1 μm, a p-type (Al_(0.3)Ga_(0.7))In_(0.5)P active layer 522 with athickness of 0.2-1 μm, an n-type (Al₀₃Ga₀₇)In₀₅P lower cladding layer523 with a thickness of 0.5-1 μm, a p+GaAs contact layer 524 with athickness of 0.1 μm, an AlAs or InGaP etching stop layer 525 with athickness of 0.1 μm, and a GaAs buffer layer 526. The LED light emittingregion 52 has a p/i/n structure and alternatively it may be an n/i/pstructure. The n+GaP only serves for the bonding layer and plane contactelectrode and, thus the primary concern is not to destroy themirror-like surface. AlAs serves as an etching stop layer and may bereplaced by AlGaAs. If the doping concentration ofp-(Al₀₃Ga_(0.7))In_(0.5)P 523 is too high, then the n+GaAs contact layer524 may be eliminated so that light is not absorbed by GaAs.

[0040] The flow diagram of adhering LED elements to a substrate is shownin FIG. 6. A glass is washed firstly (step 61). Then, an LED wafer iswashed (step 62). Next, the metal bonding agent is plated by thermalvaporizing plating (step 63). The LED element is adhered to the glasssubstrate in water, air or alcohol (step 64). The adhered structure isplaced in a wafer holding device and thermally processed (step 65). TheGaAs temporary substrate is removed from the dual substrate LED elementand then it is etched to form a plane LED element (step 66). A metalreflecting layer is plated on the rear side of the transparent glasssubstrate (step 67).

[0041] The cross sectional view of the wafer holding device of thepresent invention is illustrated in FIG. 7. The wafer holding deviceincludes a stainless steel screw 71, a graphite upper cover 72, agraphite pillar 73, a dual substrate LED element (i.e., glass and LEDwafer) 74, a graphite shim 75, and a graphite lower chamber 76. In thiswafer bonding holding device, by means of different thermal expansioncoefficients of the two materials in the holding device, the two piecesof the dual substrate LED element are pressed so as to be fused witheach other in higher temperature. The feature of the holding device ofthe present invention is that a stainless steel screw serves tosubstitute for the quartz sleeve, since the thermal expansioncoefficient of the stainless still is larger than that of graphite,during high temperature fusion, the stainless steel will apply a force.

[0042] The advantages and effects of the present invention are:

[0043] In the present invention, transparent glass is used to replacethe conventional light absorbing substrate (such as GaAs) or coloredsubstrate (such as GaP), and the substrate of transparent glass iseasily obtained. Therefore, the illumination and hue of the LED elementare improved.

[0044] In the present invention, the heating process is performed inlower temperature (about 300˜450° C.) for about 5 to 10 minutes forproviding the energy for adhering, thus the original pn junction of LEDis not affected. Further, the pollution and diffusion do not occur undersuch a lower temperature.

[0045] In the present invention, a metal is used as the bonding agent.When transparent glass is used as a permanent substrate, a metal layeris plated to the rear side of the transparent glass for enhancing theintensity of the LED.

[0046] The wafer holding device used in the present invention is shownin FIG. 7, wherein two materials with different thermal expansioncoefficients provide pressures to the LED element and transparentsubstrate, and thus the applied force can be measured by a twistingspanner.

[0047] Although the present invention has been described with referenceto the preferred embodiments, it will be understood that the inventionis not limited to the details described thereof. Various substitutionsand modifications have been suggested in the foregoing description, andothers will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

What is claimed is:
 1. A light emitting diode, comprising: a plane lightemitting region; a permanent substrate formed by a transparent material;a metal bonding agent sandwiched between said light emitting region andsaid permanent substrate; and two plane electrodes formed on said lightemitting region.
 2. The light emitting diode as claimed in claim 1 ,wherein one of said two plane electrodes is an exposed area of saidmetal bonding agent not covered by said light emitting region.
 3. Thelight emitting diode as claimed in claim 1 , wherein said permanentsubstrate is selected from glass or quartz.
 4. The light emitting diodeas claimed in claim 1 , wherein said metal bonding agent is selectedfrom the group of AuBe, In, Sn, Al, Au, Pt, Ti, Zn and Ag.